Recent developments in manganese oxide based nanomaterials with oxygen reduction reaction functionalities for energy conversion and storage applications a review

Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Review Article Recent developments in manganese oxide based nanomaterials with oxygen reduction reaction functionalities for energy conversion and storage applications: A review Yilkal Dessie a, *, Sisay Tadesse b, Rajalakshmanan Eswaramoorthy a, Buzuayehu Abebe a a b Department of Applied Chemistry, Adama Science and Technology University, Adama, Ethiopia Department of Chemistry, Hawassa University, Hawassa, Ethiopia a r t i c l e i n f o a b s t r a c t Article history: Received 22 February 2019 Received in revised form 29 June 2019 Accepted July 2019 Available online xxx In this article, a brief overview of manganese oxide nanomaterials (NMs) potential towards oxygen reduction reaction (ORR) for microbial fuel cell (MFC), bioremediations, and battery applications is discussed It's known that using non-renewable fossil fuels as a direct energy source causes greenhouse gas emissions Safe, sustainable and renewable energy sources for biofuel cell (BFC) and metal-air batteries hold considerable potential for clean electrical energy generators without the need for a thermal cycle In an electrochemical reaction system, the four-electron reduction from molecular oxygen at the air-cathode surface to hydroxide ion or water at a reasonably low overpotential was the ultimate goal of many investigations and plays a vital role in metal-air batteries and fuel cell device systems Different MnxOy nanostructured materials, from Biofunctional structural catalysts up to their electrocatalytic contributions towards ORR are discussed Brief descriptions of ORR, principle strategy and mechanism, as well as recent developments of cationic dopants and electrolytic media, effect on the air-cathode surface of manganese oxide nanocatalyst are also discussed Finally, challenges associated with platinum and carbon support platinum in improving electron and charge transfer between biocatalyst and air-cathode electrode are summarized © 2019 Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Manganese oxide nanomaterials Microbial fuel cell Bioremediations Batteries Oxygen reduction reaction Introduction Recently, the use of fossil fuels (e.g., coal, natural gas, and oil) as a direct energy source has led to a global energy crisis [1] This crisis increased focus on greenhouse gas emissions to the environment, and the limited and unstable supply of fossil fuel resources for future forecast makes unsustainability of energy resources [2] Renewable energy is thus considered as a sustainable way to reduce the current global warming crisis However, various efforts have been devoted to developing an alternative mechanism for renewable energy generation [3] Safe and sustainable futures can be ensured by making new innovations and modifications to the existing energy generation and storage device technologies The best way to this is by utilizing energy from the fuel cell, batteries, supercapacitors, etc * Corresponding author E-mail address: yilikaldessie@gmail.com (Y Dessie) Peer review under responsibility of Vietnam National University, Hanoi via oxygen reduction reaction (ORR) concerning the best mechanism for a variety of infrastructure applications [4] Among these, fuel cells and metal-air batteries are energy generators that hold considerable potential for future application and relatively clean electrical energy generators These electrochemical devices transform chemical energy from a specific fuel into electricity without the need for a thermal cycle [5] During this electrochemical reaction, a four-electron reduction reaction of molecular oxygen to either hydroxide ion or water at a reasonably low overpotential is the ultimate goal of many investigations It also plays a vital role in electrochemical energy-conversion systems in metal-air batteries and fuel cells [6] Therefore, in this review, a detailed electron transfer and potentials of the ORR mechanisms are going to be visualized to the reader with a clear and concise manner Currently, fuel cells (either biotic or abiotic) are considered to be a value add source of energy due to their high gravimetric and volumetric energy efficiency Mild operation process, zero emission and most importantly, unlimited renewable source of reactants especially biotic fuel cells are commonly known as biological or https://doi.org/10.1016/j.jsamd.2019.07.001 2468-2179/© 2019 Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/) Please cite this article as: Y Dessie et al., Recent developments in manganese oxide based nanomaterials with oxygen reduction reaction functionalities for energy conversion and storage applications: A review, Journal of Science: Advanced Materials and Devices, https:// doi.org/10.1016/j.jsamd.2019.07.001 Y Dessie et al / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx MFC It converts the chemical energy of different fuels into a useful form of electrical energy similar to that of batteries However, fuel cells not stop working until the fuels and oxidants are continuously fed The main areas of fuel cell technology are from transportation, stationary to the small-scale portable power source [7] Goswami et al reviewed a typical conventional fuel cell in which its fuel is oxidized at the anode and oxygen as an oxidant is reduced at the cathode (Fig 1) [4] This review aims to investigate the fundamentals and recent progress of manganese oxide-based electrocatalysts towards ORR capability for remediation, electricity harvesting, and energy conversion at the air-cathode electrode in the bioelectrochemical device systems According to the references cited in each section, a clear and brief summary of the principle, mechanism, and synthesis strategy for manganese oxide based NMs (bare, doped or composites, as well as, supported.) involved in the ORR is presented Oxygen reduction reaction (ORR) on biocathode ORR is the most important reaction that occurs in the cathode surface of fuel cells [4] Interconnecting living systems or their parts with simple and low-cost transition metal oxide to speed up chemical reactions on the surface of an electronic conductor for energy conversion and waste treatment is known as a biocatalyst [8] Besides their cost-effectiveness, the durability of the ORR catalysts in MFC is another major challenge, because the cathode is constantly exposed to waste effluents containing a variety of contaminants (either organic or inorganic) and microorganisms The common assembly of this cell is singlechamber and double chamber system In single-chamber MFCs (Fig 2a) the catalysts directly contact the waste and may be poisoned by intermediate products such as methanol, chloride, sulfide, etc Catalyst poisoning by such intermediates leads to high potential loss and reduced power production Furthermore, organisms can form a biofilm on the cathode surface and degenerate catalytic performance by blocking the O2 transport In some circumstances, the biofilm may serve as a biocatalyst, however, similar problems may happen in two-chamber system MFCs (Fig 2b) [8] When an appropriate and environmentally safe half potential electron acceptor (e.g., oxygen) is present in the cathode, the cell is then thermodynamically favorable, so that the electron flow across the whole system is spontaneous The most important reactions that take place in the cathode of the fuel cell are the ORR Such a reaction is a challenge in the field of catalysis and electrochemistry Normally, the reaction is a complex four-electron transfer reaction that involves the breaking of a double bond from oxygen molecule and the formation of OH-bonds through several elementary steps and intermediate species To understand such a complex process in ORR, Jiang et al developed manganese oxide catalyst using carbon as a supported material to understand its intrinsic catalytic behavior in alkaline electrolytes [9] A general classical scheme for this reaction and hydrogen peroxide (H2O2) stability as a reaction intermediate species was described in Fig [9] Therefore, other approaches due to its broad reaction pathways, a theoretical calculation are needed to shed light on the microscopic structures and processes taking place at the surface during the reaction [10] Oxygen is an ideal electron acceptor molecule for MFCs due to its high redox potential, availability, and sustainability However, the ORR is kinetically sluggish, resulting in a large proportion of potential loss known as activation loss next to concentration and ohmic losses To reduce such loss, MnO2 as a nanocatalyst has been successfully used as a cathode material in both aqueous and non-aqueous fuel cells However, most of these oxides as ORR achieved only half of that with platinum/carbon (Pt/C) electrode, regardless of structure modifications or doping with other transition metals Therefore, this review also summarizes how this different nanostructured component of MnXOY (MnO2, Mn2O3, Mn3O4, Mn5O8, and MnOOH) [8], coexists and their contributions would be clearly discussed to ORR Beyond ORR some manganese oxides such as MnO2, Mn2O3 and Mn3O4, which were synthesized by a hydrothermally technique, exhibit the best catalytic activity performance towards NO oxidation with its maximum conversion efficiency of 91.4% [11] Thermally decomposed ε-MnO2 from manganese nitrate using carbon powder as a support mixture was Fig A typical fuel cell Adapted with permission from Elsevier [4] Please cite this article as: Y Dessie et al., Recent developments in manganese oxide based nanomaterials with oxygen reduction reaction functionalities for energy conversion and storage applications: A review, Journal of Science: Advanced Materials and Devices, https:// doi.org/10.1016/j.jsamd.2019.07.001 Y Dessie et al / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx Fig Schematics of (a) single-chamber MFCs and (b) two-chamber MFCs Adapted with permission from The Royal Society of Chemistry [8] mez-Marín et al is reproduced from Ref [10] and is licensed under CC BY 2.0 (http://creativecommons.org/ Fig Reaction pathways proposed for the ORR This figure by Go licenses/by/2.0) tested for ORR in an alkaline electrolyte The complete 4eÀ reduction pathway via a plus 2eÀ reduction process were proceeds on MnO2 catalysts through involving H2O2 as an intermediate The close performance for ε-MnO2/C material in comparison with 20% (w/w) Pt/C catalyst as a benchmark was observed in the kinetic control region due to the presence of structural defects in this oxide This structurally modified catalyst thus has higher electrochemical activity for proton insertion kinetics [12] The art of these structural defects plus electrical conductivity on a-MnO2 effectively reduce active polarization followed by maximizing kinetics [13] Thermal reduction of Mn2O3 to MnO had also occurred for hydrogen production in solar energy concentration devices [14] All this catalytic activity is facilitating due to increase in activity of the surface amorphous MnOx, especially in a highly monodisperse amorphous MnXOY nanosphere [15] NiOx hybrid could tune the performance for its catalytic superiority towards reversible oxygen evolution reaction (OER) and ORR, due to the synergistic effect of NiOx and amorphous MnXOY on the surface of graphene nanosheets after being synthesized with a selfassembly method [16] Less ecological impact, low operating temperature and high energy density proton exchange membrane fuel cells (PEMFCs) have gained much attention In such a fuel-cell device, the fuel can be any of hydrogen, methanol, ethanol, or formic acid, whereas, highly electronegative oxygen molecule is chosen to receive the electrons released from the fuel on the cathode electrode Overall, between the fuel oxidation reaction and the ORR, electrons flow outside the cell to power electronic devices and protons migrate from anode to cathode inside through the Nafion membrane to complete the charge flow in the circuit [17] Despite their great potential, PEMFCs have their own serious limitations that prevent them from being scaled-up for commercial applications due to using expensive noble metal [18] ORR is a multi-electron transfer process that follows an electrocatalytic inner sphere mechanism The reaction is highly dependent on the nature of the electrode surface ORR is, in general, a suitable mechanism that employed for the formation of different oxygen-containing intermediates (such as OH, OÀ , O, H2O2 and HOÀ ) under both acidic and alkaline media [4] In an aqueous medium, ORR is highly reversible The possibility of rapidly reversible redox transformation in nanophase MnOx at room temperature triggered by changes in hydration [19] and potential surface Mn(IV)/Mn(III) redox couple [20,21] However, the appropriate mechanisms associated with ORR have not been well Please cite this article as: Y Dessie et al., Recent developments in manganese oxide based nanomaterials with oxygen reduction reaction functionalities for energy conversion and storage applications: A review, Journal of Science: Advanced Materials and Devices, https:// doi.org/10.1016/j.jsamd.2019.07.001 Y Dessie et al / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx understood in spite of extensive experimentation From this point of view, ORR is regarded as the kinetically limiting element of the electrochemical devices due to slow reaction rate This inspired the researchers to fabricate a novel mesoporous MnXOY electrocatalyst support due to their large specific surface area and unique pore structure which can permit more active sites for the contact between catalysts and electrolytes Such contact clearly demonstrated a good competitive ORR activity at the cathode in fuel cells, which can be used to reduce the loss of cell energy storage and conversion efficiency by overcoming the slow kinetics during ORR [22] Pt based materials have been used as a common ORR owing to its high electrocatalytic activity, stability and high exchange current density However, their high price/or cost, scarcity and low durability limit them from extensive commercialization In addition, the stability of Pt is low as it suffers from dissolution, agglomeration, coalescence, poisoning during fuel cell reaction conditions, and sintering which finally results in an unusual rise in overpotential for ORR, thus reducing both the active catalyst surface area and the catalytic efficiency, which leads to an undesired increase in overpotentials for fuel-cell device system, especially for ORR Therefore, exploring highly abundant, low cost and durable electrocatalysts with comparable or even higher catalytic performance than that of Pt-based electrocatalysts have become a key interest [23] Zeng et al have proved by hybridizing MnO2 with Ag4Bi2O5 that exhibits superior long-term durability and stronger methanol tolerance than commercial Pt/C for ORR in alkaline solution Based on this concept one way to make PEMFCs inexpensive and durable is to incorporate Pt more in the catalyst layers It can be done by using effective support materials (e.g., carbon materials) along with Pt nanoparticles or alloying Pt with other inexpensive metals like Co, Ni, Fe, etc [24] However, this approach didn't work well on a longterm basis due to the ever-growing price of Pt [25] Therefore, this review gives attention to non-precious metal or non-noble metal oxides (e.g., manganese oxide and its nanocomposites) based catalysts in detail as cost-effective alternatives to Pt For current ORR, Konev et al described effective electrocatalytic NMs with good properties from manganese and cobalt polymorphing films [26] Other than this mixed metal polymorphing, a highly active and new microporous based manganese porphyrin-polymer networks catalyst was successfully fabricated for ORR catalytic activity and its selectivity [27] Manganese oxide based nanomaterials Transition metal oxides (TMOs) are attractive noble metal-free catalysts for oxygen reduction application at the cathode of alkaline based membrane fuel cells or metal-air batteries [28] Theoretical and experimental studies revealed that TMO based catalysts having a spinel structure can act as a proficient cathode electrode material for energy conversion and storage devices For example, manganese mixed oxides [29], manganese-iron mixed oxide doped with TiO2 [30], layered manganese-cobalt-nickel mixed oxides [31], layered copper-manganese oxide [11], well dispersed spinel cobaltmanganese oxides [32], cubic Mn2O3-carbon [33], and bond competition control manganese oxide [34] have attracted much attention for excellent Bifunctional catalytic activity They also have higher electrical conductivity than single TMOs This unique property of TMOs facilitates better ORR performance, due to their variable oxidation states and better mixing ability into one material Comparatively, by forming a nanosized bimetallic cluster [35], and bimetallic oxide, its oxygen reduction and oxidation performance could be also performed [36] Furthermore, TMOs are commercially affordable due to their low price and high abundance, which makes them to be used often as electrocatalyst [37] The review extended more focuses on manganese oxide; one part of TMO based catalyst synthesis, principle, and mechanism activities towards ORR In the last few years, the study of nanoparticles has acquired enormous interest due to their variation in physicochemical, electronic, and morphological properties As a functional material, they can be synthesized in the nanometric scale ranges Due to changes in their structure and bonds, they have also displayed interesting electronic and catalytic properties Controlling the structure of catalysts at the atomic level provides an opportunity to establish a detailed understanding of the catalytic form-to-function and realize new non-equilibrium catalytic structures [38] Molecularlevel by itself is a factor that determines reaction mechanisms and electrocatalytic activity [39] Due to these properties, manganese oxides can be considered to be the most complex of the metallic oxide compound Afterward, manganese oxides as NMs have been studied due to their efficient uses in rechargeable lithium-ion batteries [40e42], a simple energy conversion [43], catalysts [44,45], capacitors [46], sensors [47,48], remediation [49], flame retardants [50], fire safety [51], alkaline fuel cells [52], radical scavenging and cytoprotection [53], pharmaceutically active compounds removal [54,55], biofilters [56], oxidative transformation [57], and water splitting [58] For an effective application, the synthesis of nanocrystalline manganese oxides has been used more widely within the context of solution chemistry For example, synthesis using thermolysis from organometallic precursors [59], directly mixing of potassium permanganate [60] and polyelectrolyte aqueous solutions [61], coprecipitation [62e64], room-temperature synthesis [65], hydrothermal [66,67], solvothermal [22], plant [68], biological [69e71], wet chemical method [72,73], electrospinning [74], solegel [75,76], sonochemical [77e79], microwave-assisted [80e82], complex decomposition [83], chemical reduction [84], electrochemical method [85,86], direct electrodeposition [87], sulfur-based reduction followed by acid leaching [88], are the most common and simple types of techniques to fabricate MnXOY nanocatalyst Obviously by simple thermal treatment method different forms of manganese oxides (MnO, Mn3O4, Mn2O3, MnO2, as well as the metastable Mn5O8) could be fabricated at different conditions [89] These oxides as shown from Table could coexist and progressively change one into the other during the oxidation process, which is usually controlled by the diffusion of oxygen There are also several relations between these manganese oxides [59,90] Manganese monoxide, MnO, occurs as the mineral manganosite Nanocrystals of this oxide are capped with organic ligands and highly dispersible in nonpolar solvents It can generate particle sizes between and 20 nm by controlling the reaction conditions and from to 40 nm particle sizes were controlled by changing the surfactant Structural characterization showed that the nanoparticles had core/shell structures with a thin Mn3O4 shell [90] The manganese oxide hausmannite, Mn3O4, is a black mineral that forms the spinel structure with tetragonal distortion due to a JahneTeller effect on Mn3ỵ In the Mn2ỵ(Mn3ỵ)2O4 structure, the Mn2ỵ and Mn3ỵ ions occupy the tetrahedral and octahedral sites, respectively, and Mn3O4 is ferrimagnetic below 43 K Nanocrystalline Mn3O4 has been synthesized by a number of methods Table Different manganese oxide nanostructural products obtained at different calcination temperature ranges In air, N2 or O2 In H2 In pure O2 gas 550o CÀ600o C 850o CÀ1050o C 950o CÀ1050o C MnO2 ƒƒƒƒƒƒƒƒƒƒƒ!Mn5 O8 ƒƒƒƒƒƒƒƒƒƒƒƒ!Mn2 O3 ƒƒƒƒƒƒƒƒƒƒƒƒ!Mn3 O4 o o 400 CÀ500 C MnO2 ƒƒƒƒƒƒƒƒƒƒƒ!MnO $190o C $430o CÀ470o C $390 C $450 C $510o C MnOƒƒƒƒƒ!Mn3 O4 ƒƒƒƒƒƒƒƒƒƒƒƒ!Mn5 O8 ƒƒƒƒƒ!Mn2 O3 o o MnOƒƒƒƒƒ!Mn3 O4 ƒƒƒƒƒ!Mn2 O3 Please cite this article as: Y Dessie et al., Recent developments in manganese oxide based nanomaterials with oxygen reduction reaction functionalities for energy conversion and storage applications: A review, Journal of Science: Advanced Materials and Devices, https:// doi.org/10.1016/j.jsamd.2019.07.001 Y Dessie et al / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx that produce relatively monodisperse particles 6e15 nm nanoparticles were obtained by thermal decomposition of manganese (II) acetylacetonate in oleylamine under an inert atmosphere They manipulate the particle size changing the employed reaction temperature Pure MnO with size in between 11 and 22 nm can also be obtained when a small amount of water was added to the reaction slurry; therefore, the metal-oxide phase is controlled by the presence or absence of water Mn2O3 exists in two forms, a-Mn2O3 and g-Mn2O3 Almost pure a-Mn2O3 occurs as the mineral bixbyite with black and crystallizes form in a cubic structure [90] The manganite, g-MnOOH is the most stable and abundant mineral of the three polymorphs of manganese oxyhydroxide The other two are feitknechtite and groutite Both of them have the same chemical formula, Mn3ỵO(OH) but differ with their crystal system The crystal system of feitknechtite is a hexagonal while groutite is orthorhombic The manganite crystal structure is similar to that of pyrolusite, but, in all the Mn is trivalent and one-half of the oxygen atoms are replaced by hydroxyl anions The Mn(III) octahedral are quite distorted because of JahneTeller effects In air manganite alters at 300  C to pyrolusite The metastable oxide, Mn5O8 is not a well-known compound of manganese It may be formed together with pyrolusite during the diagenetic decomposition of manganite Mn5O8 is established as an intermediate phase too, forming between MnO2 and Mn2O3 at or above 300  C The unit cells of pyrolusite, manganite, and Mn5O8 are closely related, their crystallographic axes remain in nearly the same relative orientations Mn5O8 crystallizes in a monoclinic structure containing 4ỵ mixed valences of Mn2ỵ and Mn4ỵ as Mn2ỵ Mn3 O8 [90] Doping manganese with latest transition metals such as cobalt encased within bamboo-like N-doped carbon nanotubes [91] and lanthanum at a time can increase the stability and improving the catalytic activity for ORR [92,93] One of the challenges in the synthesis of oxide nanoparticles is obtaining monodisperse nanoparticles Furthermore, it is also necessary to control the overall sizes of nanoparticles, as well as to know its exact composition In particular for manganese oxides, one of the most important challenges is to obtain a single phase, because in almost all procedures the obtained result is significant for coreeshell structures All these challenges are not easy to solve since nanoparticles are unstable during long periods of time Furthermore, since nanoparticles are highly reactive, they oxidize easily in air, losing catalytic activity and dispersibility Because of this, protection strategies are used, like capping with surfactants, organic products or inorganic membranes [90] Principle and mechanism of ORR on MnxOy surface Based on the principle of physical adsorption of oxygen molecule (O2) on manganese oxides surface due to high contact between electrolyte and active catalyst, O2 is converted to either OHÀ or H2O During this conversion, manganese oxides are known active catalysts in a given media [94] Practically, in alkaline solution, the mechanism of oxygen reduction at MnO2 catalyzed air cathode was investigated by measurements of polarization curves in a wide range hydroxide ion (OHÀ) concentration, oxygen pressures, and using different crystalline MnO2 catalysts [95] ORR at the cathode surface precedes either partially or two electron reduction pathways results in the formation of adsorbed H2O2 species and direct four-electron reduction pathways Direct four electron pathways are more desirable for ORR than the partial reduction pathway since the reactivity of H2O2 is comparatively higher than that of the stability of H2O [96] The direct conversion of O2 into H2O involves a dissociative mechanism, where the first step is the adsorption of O2 on the metal/catalyst surface followed by breaking off the oxygeneoxygen bond to give adsorbed oxygen atoms Subsequently, transfer of electrons to the adsorbed oxygen atoms in the form of hydrogen addition, yields surface-bound hydroxyl groups Further reduction and protonation of the hydroxyl group produce the H2O molecule leaving behind the metal/catalyst surface On the other hand, partial reduction of O2 follows an associative mechanism in which the adsorption of O2 on the metal surface doesn't lead to the cleavage of oxygeneoxygen This alternative two-electron reduction pathway finally generates H2O2 [17] For more clarifications, Table shows the pathways of ORR in alkaline and acidic medium [97] It is interesting that grapheneoxide-intercalated layered manganese oxide enhanced four electron transfer activity towards ORR in alkaline media at 0.8 V vs RHE [98] Because, electrodeposition of manganese oxide into a graphene hydrogel not only improves the carbon material's capacitive performance, but also affects the surface chemical environment of the graphene-oxide framework [99] Controllable growth from uniform nanoparticles with specific morphology to obtain a high active electrocatalyst is a key common problem in developing efficient energy conversion and storage devices [91,100] Even though to reduce such challenge, Sun and his coworker Liu have fabricated a nanoflake oxygen reduction ternary composite catalyst from manganese oxide and CNTs-graphene support for an elevated power performance in pilot scale manufacturing technology [101] A typical ORR polarization curve (Fig 4) is generally divided into three regions, these are kinetically controlled region, diffusion controlled region and mixed kinetic and diffusion controlled regions On the kinetically controlled region the rate of O2 reduction is slow with a small increase in the current density as decreasing potential A substantial rise in the current density is observed in the mixed kinetic and diffusion controlled area In this region, acceleration of the reaction takes place with a marked drop in the potential value In the diffusion controlled region, the current density is determined by the rate at which diffusion of the reactants occurs Quantitative analysis of the catalyst in terms of its activity can be done from the two parameters i.e., the onset potential (Eonset) and the half-wave potential (E1/2) The more positive is the potential, the more active will be the catalyst towards ORR JL denotes the diffusion limited current density [102] Achieving efficient catalysis for ORR plays an important role in energy conversion, even if manganese oxides have attracted enormous interest due to their unique catalytic properties, manganese as an element in a higher extent may cause a potential limitation to plant growth on acidic and poorly drained soils [103] Due to the high theoretical capacitance of manganese oxide nanomaterials, 1370 F gÀ1, a huge number of works is devoted to these materials [46] Due to its variable oxidation state of manganese (ỵ2, þ3, þ4, þ6 and þ7), it contains various morphologies and crystallographic forms The structural flexibility in its oxides form (e.g., available in binary oxide type or capable of incorporating) with another metal can also form a composite structures perovskite [104], three-dimensionally ordered macroporous perovskite LaMnO3 with increased specific surface area and pore volume [105], and spinels [106] Moreover, the Mn2O3 nanoparticles catalyst with ỵ3 oxidation state exhibits higher ORR activity compared to Table ORR pathways in alkaline and acidic medium [97] Electrolyte Pathway ORR Alkaline aqueous solution eÀ eÀ Acidic aqueous solution eÀ eÀ O2 þ H2O þ eÀ / 4OHÀ À O2 þ H2O ỵ e / HO ỵ OH HO ỵ H2O ỵ e / 3OH O2 ỵ 4Hỵ ỵ e / H2O O2 ỵ 2Hỵ ỵ e / H2O2 H2O2 ỵ 2Hỵ ỵ eÀ / 2H2O Please cite this article as: Y Dessie et al., Recent developments in manganese oxide based nanomaterials with oxygen reduction reaction functionalities for energy conversion and storage applications: A review, Journal of Science: Advanced Materials and Devices, https:// doi.org/10.1016/j.jsamd.2019.07.001 Y Dessie et al / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx Fig A characteristic ORR curve of an individual catalyst Adapted with permission from the Wiley publishing group [102] the Mn3O4 nanoparticles catalyst with mixed (ỵ2, ỵ3) oxidation state [107] Tang et al have reported a nanobelt bundles manganese oxide with its specific surface area of 160 m2 gÀ1 The nanobelt bundle exhibits good capacitive behavior and cycling stability in a neutral electrolyte system, and its initial capacitance value is 268 F gÀ1 [108] A nanostructured manganese oxide which is synthesized by a simple hydrothermal route at a very low temperature of 60  C using potassium permanganate as oxidant and ethanol as reductant succeeded with a maximum specific capacitance of 198 F gÀ1 [109] A flexible and binder-free cathodic electrode for electrochemical capacitors was prepared from electrodeposition of manganese oxide onto reduced graphene oxide paper From (Fig 5) hausmannite phase manganese oxide coated electrodes exhibit a promising performance at a specific capacitance of 546 F gÀ1 with current density 0.5 A gÀ1 and 308 F gÀ1 with a scan rate of mV sÀ1 in chargeedischarge and cyclic voltammetry measurements, respectively During potential cycling, phase transformation of Mn3O4 to mixed-valent MnOx was observed Consequently, MnOx nanostructures on self-standing reduced graphene oxide electrodes have succeeded with 154% capacitance retention at 10,000 cycles from cyclic voltammetric data [110] In order to fully leverage their potential application, a precise control over particle size, surface area, and Mnxỵ oxidation state properties is required Here, the inverse micelle solegel method which is categorized by the heat treatment can control such properties followed by keeping tenability and crystallinity [111] and calcination of Mn(II) glycolate nanoparticles using polyol technique was used to synthesize a mesoporous a-Mn2O3, Mn3O4, and Mn5O8 nanoparticles The authors conclude that these different oxidation states of manganese oxide nanoparticles using such route can facilitate their actual structuraleproperty relationship In situ X-ray diffractometer measurements suggested that different MnOx phases were observed From the analysis, it is conclude that a complete time and temperature dependent phase transformations were occurred successfully from Mn(II) glycolate precursor oxidation to a-Mn2O3 via Mn3O4 and Mn5O8 in O2 atmosphere From sweep voltammetry measurements, mesoporous a-Mn2O3 showed a good kinetic enhancement potentials for ORR in aprotic media [59] In the electrochemical ORR, H2O2 has been detected as a reaction intermediate on TMO and other electrode materials Hence, the electrocatalytic and catalytic reactions of H2O2 on a set of manganese oxides such as Mn2O3, MnOOH, LaMnO3, MnO2, and Mn3O4, were studied All of these different crystal structures were adopted to shed light on ORR mechanisms Among MnO2 has attracted great attention due to its high catalytic activity, thermal stability, facile synthesis with low-cost materials and availability in various crystal morphologies [112] Kinetic modeling and experiment objective correlates the differences in the ORR activity to the kinetics of the elementary reaction steps displayed that the catalytic activity of Mn2O3 was better in the ORR due to its high catalytic activity both in the reduction of oxygen to H2O2 detection with its unique crystal structure and reactivity shown from the tentative mechanisms (Fig 6) [47] Previously, aggregates of gold nanoparticles (AuNPs) on manganese dioxide nanoparticles (nano-MnO2) was developed for better H2O2 amperometric sensing [113] Electrodeposited manganese oxide in the average size range of 21e40 nm was identified with different phases (MnO, MnO2, and Mn3O4) for H2O2 detection [114] Till now, a represented divalent alkaline-earth metal ion or trivalent rare-earth metal ion (such as perovskite (AMnO3) or spinel (AMn2O4) structure) adopted by Mn-based oxides displays an efficient ORR activity [115] To improve the activity of Mn-based oxides oxygen defects have been introduced by thermal reduction which reduces Mn4ỵ to more active Mn3ỵ, and improves the electrical conductivity However, the overall ORR activity of Mn-based oxides has been still higher than that of Pt/C To design an active ORR catalyst the oxidation state of manganese centers is critical The intermediate species for this reaction is Mn3ỵ, which plays a significant role in the success of catalytic activity in ORR To boost the catalytic activity, numerous approaches have been made to generate the active Mn3ỵ species The high catalytic activity of Mn3ỵ species is attributed to the presence of one electron resulting in JahneTeller (JeT) distortion [116] Therefore, to achieve high Please cite this article as: Y Dessie et al., Recent developments in manganese oxide based nanomaterials with oxygen reduction reaction functionalities for energy conversion and storage applications: A review, Journal of Science: Advanced Materials and Devices, https:// doi.org/10.1016/j.jsamd.2019.07.001 Y Dessie et al / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx Fig (a) Cyclic voltammograms of Mn3O4/rGO with different mass loads at 20 mV sÀ1; (b) Cyclic voltammograms of Mn3O4/rGO deposited at À1.1 V with a fixed charge of 500 mC at different scan rates; (c) Capacitance retention of the film as a function of cycle number; (d) Nyquist diagram of Mn3O4/rGO along with the equivalent circuit to fit the experimental data (solid line represents the fitted curve) Adapted with permission from Elsevier [110] Fig A tentative mechanism for the oxygen reduction/H2O2 reactions Adapted with permission from the Wiley publishing group [47] specific ORR activities, the presence of Mn3ỵ with some Mn4ỵ is the key in perovskites Cao et al also showed the dependence of electrochemical activity on the crystalline structure of manganese oxides The obtained result shows that, the ORR current of different MnO2 catalysts increase in the following order: b-MnO2 < l-MnO2 < gMnO2 < a-MnO2 < d-MnO2 [95] The specific morphology and crystalline structure effect on a-MnO2 nanowires, a-MnO2 nanorods, b-MnO2 nanowires, and b-MnO2 nanorods have been successfully synthesized via a hydrothermal process, and their microstructures and electrocatalytic activities were investigated for ORR Among the four different types of one-dimension MnO2, the a-MnO2 nanowires exhibited significantly larger electrocatalytic property than the others This is due to the highest electron transfer Please cite this article as: Y Dessie et al., Recent developments in manganese oxide based nanomaterials with oxygen reduction reaction functionalities for energy conversion and storage applications: A review, Journal of Science: Advanced Materials and Devices, https:// doi.org/10.1016/j.jsamd.2019.07.001 Y Dessie et al / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx number, which may contribute to the special crystalline structure and larger specific surface area As a result, more active sites could be exposed in the three-phase (electrolyte, oxygen, and catalyst) interfaces during the whole reaction process and thus enhance the ORR catalytic performance [117] Lower valence state manganese oxides are also targeted for ORR; Mn3O4 is rich in electrochemical properties due to the mixed valence of Mn However, because of its poor electrochemical structural stability and low electrical conductivity, its use as ORR catalyst is diminished Goswami et al have reviewed carbon-coated tubular monolayer superlattices (TMSLs) of hollow Mn3O4 NCs (h-Mn3O4-TMSLs) by exploiting the structural evolution of MnO nanocomposites They have characterized the catalyst by various techniques From transmission electron microscopy and x-ray diffractometer characterization result (Fig 7a), it can be seen that the average diameter of the particles is 18 nm Fig 7b and c shows the effectiveness of this in achieving highquality nanocrystal monolayers within anodized aluminum-oxide channels The XRD pattern of MnO@Mn3O4@AAO mainly ascribed to the cubic MnO phase shown in Fig 7d The presence of Mn2ỵ and Mn3ỵ can be clearly showed from x-ray photoemission spectroscopy result (Fig 7e) [4] Due to lack of accepted protocols for its precise catalytic activity measurement, Mn/polypyrrole (PPy) nanocomposite has a unique quantitative assessment for the ORR electrocatalytic activity in alkaline aqueous solutions based on the rotating risk electrode method [118] Hazarika et al have synthesized mesoporous cubic Mn2O3 nanoparticles supported on carbon (Vulcan XC 72-R) for both ORR and OER They have shown that the ORR activity of Mn2O3/C material is much better compared to the commercially available Pt/ C and Pd/C in alkaline media However, Mn2O3 without the carbon support shows less ORR activity compared to Mn2O3/C, Pt/C and Pd/ C From the parallel fitting lines of the KeL plots the average electron transfer number was found to be z1.2 and z4.1 for Mn2O3 and Mn2O3/C, respectively The high catalytic activity is due to the synergistic influence of Mn2O3 and carbon interface They have also proved that Mn2O3/C is quite stable up to 1000 cycles and the reaction follows a 4-electron pathway for ORR [33] The combination of Mn oxide with other TMOs (such as Co, Fe, Cu oxides, etc.) provides excellent ORR activity useful for a range of applications The high catalytic activity is due to the synergistic effect of the mixed TMOs Li et al have prepared ultra-small cobalt manganese spinels using simple solution-based oxidation precipitation and insertion-crystallization process at the mild condition They have studied the catalyzation of nanocrystalline spinels for ORR Furthermore, strongly coupled carbon support spinel nanocomposites exhibit similar activity except superior durability to carbon support platinum catalyst [42] Even structural and surface changes can happen on manganese oxide after modification using cobalt during activation within ethanol steam reforming reaction [119] Effect of cationic dopants and media on MnOx structure A series of calcium-manganese oxides (CaMnO3, Ca2Mn3O8, CaMn2O4, and CaMn3O6) and the detailed investigation of their electrocatalytic properties were reported through a simple Fig (a) TEM image of octahedral MnO NCs used for constructing h-Mn3O4-TMSLs; Cross section SEM images of MnO NC monolayers self-assembled within the AAO template having (b) circular and (c) hexagonal channels, respectively; (d) XRD patterns of MnO@Mn3O4@AAO and h-Mn3O4-TMSLs, respectively The blue asterisks denote the reflections of Mn3O4; (e) High-resolution Mn 2p XPS spectra of MnO@Mn3O4 NCs and h-Mn3O4-TMSLs, respectively Adapted with permission from Elsevier [4] Please cite this article as: Y Dessie et al., Recent developments in manganese oxide based nanomaterials with oxygen reduction reaction functionalities for energy conversion and storage applications: A review, Journal of Science: Advanced Materials and Devices, https:// doi.org/10.1016/j.jsamd.2019.07.001 Y Dessie et al / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx calcination route using Ca1ÀxMnxCO3 solid-solution precursors The parallel formation of highly crystalline CaeMneO porous microspheres with similar textures and its ORR catalytic activities of the synthesized CaeMneO compounds were compared with MnOx in alkaline conditions and Pt/C was used as a benchmark The experimental and theoretical study demonstrated that the surface Mn oxidation state and crystal structure are influential factors to determine CaeMneO electrocatalysts activity In recent years, CaeMneO has captured strong scientific interest due to exceptional catalytic activities, inexpensive method of synthesis, abundant, and environmentally benign nature of elements In particular, the catalytic properties of CaeMneO systems have been reviewed as a potentially useful new tool in addressing energy and environmental problems [120] Structurally, cation dopants on a-MnO2 have a vital role for ORR Hydrothermally synthesized nickel-doped a-MnO2 nanowires (Nia-MnO2) at different weight ratio in general, had higher n values (n ¼ 3.6), faster kinetics (k ¼ 0.015 cm sÀ1), and lower in charge transfer resistance (RCT ¼ 2264 U at the half-wave) values than MnO2 or Cu-a-MnO2 This was happened due to the effective surface defect functionality between nickel and manganese Therefore, the overall catalytic activity for Ni-a-MnO2 trended with increasing Ni content, i.e., Ni-4.9% > Ni-3.4% was increasing [121] Off course other than cation doping, surface topography or shape of a catalyst have had different catalytic activity towards ORR For example, Affandi and Setyawan reported that nanocatalysts were prepared electrochemically from KMnO4 precursor in different media conditions at a temperature of 60  C The result surface morphology was nanorod at pH ¼ 0.2 and nanoflake at pH ¼ 9, respectively The acidity of the solution systematically influenced the particle morphology As shown in Fig 8, the particles had nano-rod morphology at a very acid solution whereas they had a nanoflake shape at the base condition XRD revealed that the particle generated at very acid condition was a-MnO2 while at basic condition MnO2 was amorphous So, the electrocatalytic activity for nanorod and nanoflake MnO2 towards ORR of the materials was studied in oxygen saturated 0.6 M KOH solution Thus, the number of electrons transferred during ORR was 2.23 and 1.75, respectively This result suggested that nanorod MnO2 particles was exhibited better ORR activity than nanoflake MnO2 [86] Surface manganese valence of manganese oxides exhibits better catalytic activity toward the ORR than those with lower Mn valences on the activity of ORR in alkaline media [122] A porous spinel-type of magnetic iron-manganese oxide nanocubes with a hollow structure deposited on the reduced graphene oxide nanoflakes nanocomposite [123] and (CoMn2O4 and MnCo2O4) spinel microspheres reflect a high efficient catalyst for OER, as well as for the ORR The as-prepared cubic MnCo2O4 displays better OER activity compared to the tetragonal CoMn2O4 material in an alkaline medium However, the tetragonal CoMn2O4 material display better ORR activity and stability compared to cubic MnCo2O4 and also Pt catalysts Spinels structural features such as microspherical morphology and their unique porous results in the higher catalytic activity and stability of the material [106] Its spinel arrangements are continued because manganese oxides structure flexibility under working conditions remains a great challenge for identifying their active structures [124] Liang et al reported a manganese-cobalt spinel MnCo2O4/graphene hybrid is highly efficient in electrocatalyst for ORR in alkaline conditions They have suggested from the X-ray absorption near edge structure of Co Ledge and Mn L-edge that substitution of Co3ỵ sites by Mn3ỵ resulting in higher catalytic sites that enhance the ORR activity compared to the pure cobalt oxide hybrid Mechanically, such hybrid material possesses greater activity and durability than the physical mixture of nanoparticles and N-rmGO and the MnCo2O4/ N-graphene hybrid displays higher ORR current density and stability compared to Pt/C in alkaline solutions at the same mass loading [109] In addition, the use of manganese ore as an oxygen carrier has recently gained interest, primarily due to the combination of low cost and moderate to high reactivity The possibility of an oxygen uncoupling reaction enhancing reactivity may be an additional advantage [125] Fig Tafel plots of electrodes for KMnO4 solution at pH 0.2 and pH electrolysis Adapted with permission from the Ceramic Society of Japan [86] Please cite this article as: Y Dessie et al., Recent developments in manganese oxide based nanomaterials with oxygen reduction reaction functionalities for energy conversion and storage applications: A review, Journal of Science: Advanced Materials and Devices, https:// doi.org/10.1016/j.jsamd.2019.07.001 10 Y Dessie et al / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx Applications of manganese oxide nanomaterial 6.1 Microbial fuel cells (MFCs) The need to reduce the costs of renewable energy conversion sources such as bioelectrochemical systems (BES) has pushed the research towards alternative cathodes performing ORR to maintain a catalytic efficiency close to that of platinum or platinum-based catalysts [126] Conventional MFCs set up consist of biological anodes and abiotic cathodes Abiotic cathode usually requires a catalyst or an electron mediator to achieve high electron transfer, increasing the cost and lowering operational sustainability Such disadvantages can be overcome by low cost biocathodes, which use microorganisms to assist cathodic reactions The classification of biocathodes is based on which terminal electron acceptor is available For aerobic biocathodes with oxygen as the terminal electron acceptor, electron mediators, such as iron and manganese are first reduced by the cathode (abiotically) and then reoxidized by bacteria Anaerobic biocathodes directly reduce terminal electron acceptors, such as nitrate and sulfate, by accepting electrons from a cathode electrode through microbial metabolism [127] Manganese by itself as a manganese peroxidase enzymes as catalyst could apply as an enzymatic electrode in the cathode chamber of an MFC Its output power density was 100% higher than that for the conventional graphite electrode As a biocathode, its activation overpotential loss was diminished during H2O2 reduction (Fig 9) [128] Air cathode (open air at the cathode) in a single chambered MFC is the one which uses oxygen (O2) as a direct electron acceptor species and is reduced by the electrons coming from the anode and the protons via the membrane into water However, ORR on the surface of air-cathode is one of the main drawbacks in MFCs The reaction kinetics is limited by an activation energy barrier (activation polarization loss) which impedes the conversion of oxygen into the reduced form at the cathode surface; hence, it requires an efficient and effective catalyst for ORR Even at the laboratory scale platinum (Pt) is the most practically used catalyst for the ORR in MFC But due to its special case, Pt-based catalyst in large scale air cathode MFCs is limited due to high-cost and dissolution at a short lifetime in a given media In addition, other external factors also directly affect MFC performance Therefore, to reduce cost followed by increasing ORR rate, synthesizing a non-precious a-MnO2 catalyst using a hydrothermal method is being a unique strategy for aircathode application [67] Results from Table found that 28.57 mg cmÀ2 a-MnO2 was going to be an optimum catalyst load with 13.40 mW power output Later, nanostructured Mn2O3/Pt/CNTs was also used as a selective electrode for ORR and membrane less micro-direct methanol fuel cells (DMFC) in alkaline media Interestingly, during the cell reaction, there is no activity for methanol oxidation reaction, in contrast with Pt Even the bilayer cathode was tested in this membrane less micro fuel cell under mixedreactant conditions, producing an open circuit voltage (OCV) with 0.54 V and a maximum power density of 2.16 mW cmÀ2 [129] Tan et al have manufactured MnO containing mesoporous nitrogen-doped carbon (m-N-C) nanocomposite which was lowcost non-precious metal catalysts that perform high ORR in alkaline solution with four-electron transferred per molecule This nanocomposite involves the one-pot hydrothermal synthesis of Mn3O4@polyaniline core/shell nanoparticles from a mixture containing aniline, Mn(NO3)2, and KMnO4, conducting polymer with metal precursors; and followed by heat treatment to produce Ndoped ultrathin graphitic carbon coated MnO hybrids partial acid leaching of MnO The composite exhibits superior stability and methanol tolerance to commercial Pt/C catalyst, making it a promising cathode catalyst for alkaline containing methanol fuel cell applications The synergetic effect between MnO and N-doped carbon described, provides a new route to design advanced catalysts for such energy conversion device [130] Later, a 4ỵ oxidation states of MnO2 nanostructured material was fabricated by hydrothermal technique; it was potentially applicable as cathode catalyst in MFC due to their unique properties Hydrothermally synthesized MnO2 (HSM) was one-dimensional nanorod structured that accomplish a noticeable oxygen reduction peak current due to high Table Maximum current and power produced by the MFCs with different catalyst loadings Catalyst loadings (mg cmÀ2) Maximum current (mA) Maximum power (mW) 14.28 21.43 28.57 68.58 98.91 130.14 4.14 7.26 13.40 Fig Open circuit voltageetime curve of MFC after H2O2 addition to the cathode in the presence of MnP Adapted with permission from the World Renewable Energy Congress [128] Please cite this article as: Y Dessie et al., Recent developments in manganese oxide based nanomaterials with oxygen reduction reaction functionalities for energy conversion and storage applications: A review, Journal of Science: Advanced Materials and Devices, https:// doi.org/10.1016/j.jsamd.2019.07.001 Y Dessie et al / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx aspect ratio, multivalve surface topography, and positivity nature than naturally synthesized method (NSM) of MnO2 When the HSM was employed as the cathode catalyst in MFC with leachate fed, it produced 119.07 mW mÀ2 power densities and delivered 64.68% efficiency than that of NSM at the same environmental conditions Furthermore, HSM proceeds its catalytic activity via a four-electron pathway while NSM was via a two-electron pathway towards ORR in alkaline solution; the HSM was more positive onset potential than NSM [131] The same year publicized research reported that maximum power density of the MFC equipped with electrodeposited MnO2 on activated carbon (AC) air cathode was 1554 mW mÀ2, which was 1.5 times higher than the control cathode [132] Continually, hydrothermally synthesized nanoflowers, nanorods, and nanotubes catalysts at different run times were effectively demonstrated for ORR at air cathode Cyclic voltammetry and linear sweep voltammetry investigations indicated that all of the MnO2 nanostructures can catalyze the ORR at different catalytic activities However, only nanotubes are appeared to possess the highest catalytic activity, with a more positive peak potential shift, as well as, a larger ORR peak current In MFC a maximum power density of 11.6 W/m3 was recorded using nanotube as a cathode electrode The results of this study demonstrated that nanotubes are ideal crystal structures for MnO2 and that they offer a good alternative to Pt/C for practical MFC applications [43] The other catalytic performance of nanostructured MnO2 is also existed in the form of nanoflowers, which has high beneficial structural features for fabricating stable and cost-effective electrocatalytic activity when hybridized with sulfonated graphene sheets (denoted as d-MnO2/ SGS) using simple hydrothermal method [133] Although a novel nanocomposite from manganeseepolypyrroleecarbon nanotube (MnePPyeCNT) was synthesized and demonstrated as an efficient and stable cathode catalyst for ORR in air-cathode MFCs (Fig 10) Its electrocatalytic capability of this novel material in neutral electrolyte media has been investigated by cyclic voltammetry and the data showing that MnePPyeCNT can catalyze ORR with quite good activity; this is possibly due to manganese-nitrogen active sites It has been found that an efficient and stable performance with maximum power density of 169 mW mÀ2 and 213 mW mÀ2 were recorded at the loading of mg cmÀ2 and mg cmÀ2, respectively, with comparable to platinum/carbon black (Pt/C) catalyst as a benchmark in MFCs devices [18] 11 In addition to the protic and aprotic solution, functionalized manganese oxide/carbon nanotubes (MnO2/f-CNT) nanocomposite is a good catalyst for ORR in neutral solution From this study, the unique interaction between MnO2 and f-CNT was enhanced for fast electron transfer process during ORR Raman spectra result proved that more surface defect was formed after functionalization with manganese dioxide The XRD spectra showed crystallinity existence in MnO2/f-CNT catalyst During the test from the power curve, as shown in Fig 11, the higher maximum power density was achieved at 520 mW mÀ2 for MnO2/f-CNT compared to CNT (275 mW mÀ2) and f-CNT (440 mW mÀ2) alone in a MFC Moreover, for better performance clarification a 28.65% coulombic efficiency and 86.6% chemical oxygen demand (COD) removal efficiency was recorded throughout the analysis [134] Finally, a low cost iron phthalocyanine (FePc)-MnOx composite catalyst was prepared for ORR in the cathode of membranelles single chamber MFC and more power using composite FePcMnOx/ carbon monarch 1000 air cathodes (143 mW mÀ2) on the setup was generated than commercial platinum catalyst (140 mW mÀ2) and unmodified FePc/carbon monarch 1000 (90 mW mÀ2) [135] Sindhuja et al have reported the possibility of synthesizing phase specific a-MnO2 by MFC for energy storage with simultaneous power conversion applications [136] 6.2 Bioremediation The demand for new technologies to accelerate the decontamination of contaminated sites and reduce the costs of these technologies is increasingly growing Recently, the use of NMs as an innovative method to contaminated site remediation has received greater attention [49] The many techniques used in the remediation of wastes fall under the major categories of physical, biological, photolytic, chemical, and bioelectrochemical Of these, BES is regarded as a promising alternative biological wastewater treatment technology, as the energy recovered could offset partial energy consumption during the process MFC is a typical BES which can remove organic pollutants through oxidation and reduction at anode and cathode electrode system [55], respectively To replace carbon supported platinum catalysts, manganese oxide is a promising alternative electrocatalyst for ORR to reduce cost and minimize sluggish ORR rate at air cathode during wastewater treatment and power generation in MFC The MnO2 Fig 10 Schematic representation of the preparation procedure for manganeseepolypyrroleecarbon nanotube composite Adapted with permission from the Elsevier [18] Please cite this article as: Y Dessie et al., Recent developments in manganese oxide based nanomaterials with oxygen reduction reaction functionalities for energy conversion and storage applications: A review, Journal of Science: Advanced Materials and Devices, https:// doi.org/10.1016/j.jsamd.2019.07.001 12 Y Dessie et al / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx Fig 11 (a) Polarisation and power curves of MFC equipped with CNT, f-CNT, and MnO2/f-CNT as a cathode catalyst in dual chambers MFC test and; (b) coulombic efficiency and COD removal of MFC with different cathode catalysts Adapted with permission from Elsevier [134] catalyst prepared by hydrothermal, solegel, and a wet impregnation method was used to catalyze palm oil mill effluent in the anode chamber with active anaerobic sludge Cyclic voltammogram results showed that platinum support MnO2 could effectively catalyze ORR at air cathode electrode and generates a maximum power density of 165 mW mÀ3, which was higher than MnO2 catalyst alone (95 mW mÀ3) [76] Decomposition of organic compounds via oxidative reactions using microorganism as a catalyst is increasingly examined as an alternative approach to wastewater treatment and soil remediation for decomposing organic pollutants but are limited by an external constraints, such as denaturation and cost Recently, manganese oxide NMs has been found to exhibit a pollution control application with reactivity similar to laccase and phenol oxidase containing enzymes The substrates of many organic contaminants can decompose and change color during treatment Oxidation of sulfonated aromatic compound, such as 2, 20 -azinobis-(3-et hylbenzthiazoline-6-sulfonate) has been employed in the presence of laccase microorganism to assess its activity In comparison to such microorganism, certain manganese oxides (MnOx) can utilize substrate oxidation via single electron transfer mechanism, while the resultant reduced MnOx red can be reoxidized to MnOx by the dissolved oxygen molecules The dissolved molecular species under optimum conditions could be reduced to water, this clearly leading to the net electron shuttling formation from substrates to oxygen [137] Wang et al have reported the laccase-like reactivity of nanostructured manganese oxides with diverse crystallinity, including a-, b-, g-, d-, and 3-MnO2, and Mn3O4 [137] From the reaction rate behaviors, researchers have examined g-MnO2 exhibits the best performance Simultaneous approaches for generating electricity and remediation from waste and biomass using MFCs have been devoted to clean and renewable energy sources Some significant research has been dealing with the cathode reaction and catalysts for ORR; which remains a major factor in the design of low-cost MFCs Indeed, poor kinetics of ORR at neutral pH and low temperatures have hindered the improvement of MFC performances Platinum is known to be the best catalyst for the ORR in acidic and alkaline media and is the most commonly used catalyst in the MFC [7] However, Pt cost is prohibitive to economic MFCs and platinum-free electrocatalysts represent a necessary alternative Roche et al have thus investigated the performance of operating MFC using (MnO2/C) catalysts formed chemically on AC as cathode materials The test was performed at neutral and alkaline pH [44] Later, the electrocatalytic activity of MnOx modied with Cr3ỵ, Fe2ỵ, Co2ỵ [138], Zr4ỵ [139], and metal (e.g., Ni, Mg) ion [44] have been performed for better efficiency through doping The result shows that, and all the catalysts improve O2 adsorption for superior catalytic activity towards the ORR Beyond hybridization with metals, electrodeposition of MnO2 on polymers like polypyrrolecoated stainless steel (SS) greatly improved MFC efficiency (better in power generation and best for remediation) with almost 100% COD removal and maximum power density of 440 mW mÀ2 This indicates that MnO2/PPy-coated SS316 is one of the most promising electrode materials applicable for remediation and power generation in MFCs device system [140] 6.3 Batteries High activity with low cost Bifunctional electrocatalysts for ORR is a current strategy research priority in the improvement of energy conversion and storage devices [91] Unique structural spinel-type oxides electrocatalysts are technologically important in electrochemical energy conversion and storage fields Ultra-small CoeMneO spinels were synthesized at moderate condition for such applications Its phase and composition co-dependence showed better catalytic activity towards ORR and OER Furthermore, its synthesis strategy at optimum condition allowed for homogeneous and strongly coupled nanocomposite formation, which exhibit a comparable and superior activity in comparison to Pt/C, and it served as an effective and capable catalysts to construct and rebuild a rechargeable Zn [141] and Li [42] air batteries Today in comparison to common rechargeable batteries like lead-acid and Li-ion batteries [142], metal-air batteries [143] represent a low cost, low pollution, lightweight, relatively high specific capacity/energy density, and considered as a safe technology However, among metal-air batteries secondary zinc-air, batteries are based on an aqueous alkaline electrolyte are still under development due to the short life cycle of their electrodes Regarding the Bifunctional air electrode, the large overpotential (DV) between OER and ORR reduces the cycle life limits secondary zinc-air batteries performance The search for low-cost catalysts with sufficient manganese oxides NMs exhibits a promising electrocatalytic for OER and ORR under alkaline conditions and possess many advantages such as abundance in natural ores, low toxicity, low-cost, and environmental friendliness Due to the presence of more active centers, e.g., edges and kinks and an increase of reoxidation efficiencies on carbon powder-based electrodes, as well Please cite this article as: Y Dessie et al., Recent developments in manganese oxide based nanomaterials with oxygen reduction reaction functionalities for energy conversion and storage applications: A review, Journal of Science: Advanced Materials and Devices, https:// doi.org/10.1016/j.jsamd.2019.07.001 Y Dessie et al / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx 13 Table Summary of parameters compared with the reviewed data on manganese oxide based materials Precursors Synthesis method Electrode Electrolyte Cell voltage Number of cycles Energy density Discharge capacity Mn(NO3)2, Co(NO3)2 CoeMn KCl 0.800 V 12,000 650 Wh kgÀ1 e KMnO4, SnCl2 Two step oxidation precipitation and crystallization Wet method SnO2/Mn2O3 e 0.750 V 400 e 1365 mA h gÀ1 EMD Thermal method a-MnO2 KOH 1.420 V 200 e 10 mAh cmÀ2 KMnO4 Hydrothermal method Mn2O3/Mn3O4 KOH 0.830 V e e **1.75 Mn(CO3)2, AgNO3 Pyrolysis AgeMnO2 KOH 0.859 V e *204 **199 KMnO4, Ce(NO3)3$6H2O Modified redox synthesis CeeMnO2 KOH 0.900 V 10 *348.8 **100 Note: EMD ¼ Electrolytic manganese dioxide, * ¼ Power Density (mW cm À2 ), ** ¼ Current Density (mA cm À2 Reference [42] [142] [144] [146] [147] [148] ) as an overall decrease of potential gaps between OER and ORR enhance the activity of a-Mn2O3/C compared to pure carbon powder alone, Mn3O4/C and Mn5O8/C electrodes [59] Mainar et al produced an efficient durable and low-cost air cathode for a secondary zinc-air battery with low polarization between the ORR and OER [144] We would like to emphasize that change in surface behavior of air-cathode NMs electrode for its novel and superior electrocatalytic activity during ORR activity This ORR activity are systematically investigated by KouteckýeLevich plots in the diagnoses of charge-transfer mechanisms at rotating disk electrodes [145] From morphological studies, due to its anisotropic structures manganese oxide exhibits better significance in ORR kinetics with an improved onset potential of ỵ0.83 V versus reversible hydrogen electrode in alkaline media [146] When the smaller diameter of silver nanoparticles is anchored on the surface of a-MnO2 would result for strong interaction between Ag and MnO2 components The electrochemical tests show that the activity and stability from 50% AgeMnO2 composite catalyst toward ORR are greatly enhanced Moreover, the peak power density in the aluminum-air battery with 50% AgeMnO2 can reach up to 204 mW cmÀ2 [147] Cerium ion intercalated birnessite-type manganese oxide (d-MnO2) dispersed on CeeMnO2/C has high-efficient ORR electrocatalytic ability and exhibits excellent in long-term stability with current retention of 96.4% after aging for 40,000 s in aluminum-air battery [148] Therefore, Table represents the general summary and performance of manganese oxide based nanomaterials modifications with oxygen reduction reaction functionalities for battery application nanorods, nanoflower, nanotube, nanoflake, etc as an alternative ORR catalyst have been reviewed A wide range of MnxOy based catalysts using the hydrothermal synthesis methods followed by calcination is found to exhibit higher ORR activities but their designing as a catalyst with superior ORR activity still remains a difficult task Due to this problem, various researches from their scientific reports are trying to analyze the mechanism of these oxides by modifying manganese oxides with other materials in the form of composites This can lower the challenges found in Pt catalyst and may also provide some extra stability Manganese oxides with spinel and perovskite structures as advanced catalysts would be a recommended strategy to increase an energy conversion and storage devices with facilitating reaction kinetics, reaction mechanisms, and reaction pathways of ORR in aqueous alkaline media Therefore, for the future, a complete and noticeable manganese oxide based catalyst will be likely the key to unlock superior ORR activity This future direction may help the researchers to address all the challenges that effectively facilitate the electron transfer between the active sites and adsorbed oxygen molecules This intention illustrates that how a facile pathway could improve a catalytic activity of mixed valence metal oxides Conclusion and future outlook The authors gratefully acknowledge Adama Science and Technology University for financial support to this review article The ORR potential system continues to play an important role in energy conversion and storage applications, such as BFC and metalair batteries Pt is the most common type of catalyst for ORR within various energy conversion and storage systems, however, due to its high cost, an alternative manganese oxide nanocatalyst is going on to reduce the use of Pt in ORR potentials To reduce the cost and enhance the ORR performance, much work has been focused on manganese oxides as catalysts due to their varied valence state, crystallite structure, exceptional electrical and redox properties, studies of which are helpful to understand their behavior and mechanism So far, to increase the ORR performance, various strategies are employed, such as mixing with other metals as a support, doping, etc The performance of ORR can also be improved within a better electrocatalytic performance towards the fuel cell, batteries and bioremediation technologies due to its synergy effect In this review, some of the most important trends 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nanomaterials with oxygen reduction reaction functionalities for energy conversion and storage applications: A review, Journal of Science: Advanced Materials and Devices,